Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Dispersion compensating devices are important components of optical communication systems. Chromatic dispersion occurs when signal components of different wavelengths are subject to different propagation delays. Such dispersion can distort a transmitted pulse and deteriorate the information content of a signal channel. Dispersion compensating devices equalize the propagation delays among the different wavelength components and maintain the quality of the transmitted information.
All-pass filters are useful in compensating optical communication systems. An all-pass filter (APF) is a device which substantially equalizes phases among the different wavelength components of a signal with minimal modification of the amplitude response. In principle APFs can correct phase distortion in an optical communication system without adding amplitude distortion.
The present applicants have disclosed an APF suitable for optical communication systems in U.S. patent application Ser. No. 09/183,980 entitled "All-Pass Optical Filter" filed Oct. 30, 1998, which is incorporated herein by reference. An exemplary embodiment of this APF comprises a length of relatively straight optical waveguide coupled to one or more co-planar ring optical resonators. The ring resonators may themselves be coupled to additional ring resonators to form a multiple-stage APF. A light pulse traveling in the straight waveguide couples in part to the ring resonator. After transit around the ring, the coupled light in turn couples back to the waveguide. Interference between light from the resonator and light transmitted on the waveguide produces a frequency dependent time delay that compensates dispersion. The performance of the APF depends primarily on three parameters: 1) the ring radius, 2) the coupling strength between the ring and the waveguide and 3) the number of rings. The ring radius determines the free spectral range (FSR) of the device. The response is periodic in frequency, and the period is the FSR. The coupling strength determines the maximum group delay and the bandwidth of the enhanced delay. And a larger number of rings can generate more delay.
This APF works well for many applications, however with the ever increasing demand for greater bandwidth in communication systems, APFs of even greater bandwidth are desired. Greater bandwidth implies smaller rings. But smaller rings mean a reduced ring radius which reduces waveguiding efficiency. Increasing the relative index of the ring ##EQU1##
where .DELTA.n=n.sub.cladding -n.sub.core and n=1/2 (n.sub.cladding +n.sub.core)) ameliorate this loss in efficiency, but increased ring index requires the ring be placed impractically close to the waveguide to achieve the required coupling strength. Accordingly, there is a need for a new APF design to provide dispersion compensation over an increased bandwidth.